The following relates to the nuclear power generation arts, nuclear reactor safety arts, nuclear reactor control arts, and related arts.
During normal operation of a nuclear reactor, the nuclear chain reaction (and hence the thermal output of the reactor) is controlled to maintain temperature and pressure of the coolant water in the reactor pressure vessel in a normal range. This control is achieved using mechanisms such as a control rods system, control of steam generator feed water flow, adjusting a concentration of soluble boron neutron poison in the coolant water, or so forth.
The steam generator acts as the heat sink for the nuclear reactor. If a pipe rupture interrupts feed water flow into the steam generator or steam flow out of the steam generator, a feed water flow is lost, or so forth, this heat sinking is compromised. In such a loss of heat sinking event, pressure and temperature of the coolant in the reactor pressure vessel rise, and this must be counteracted to maintain the nuclear reactor in a safe condition.
To this end, the control rods are inserted (scrammed) to extinguish the nuclear chain reaction. However, the reactor core continues to output residual heat due to radioactive intermediate products of the nuclear chain reaction. Passive cooling systems reliant on a closed-loop evaporation/condensation cycle may be employed to remove this residual heat.
If the passive cooling systems are unable to keep up with the residual heat output from the shut-down reactor core, then the temperature and pressure of the coolant inside the pressure vessel will continue to rise. In this circumstance, an emergency core cooling system (ECCS) is brought online to depressurize the pressure vessel by venting (into a suitable condenser, or into the surrounding radiological containment structure, or other suitable structure) while maintaining sufficient water flow into the reactor pressure vessel to avoid exposure of the nuclear reactor core.